A Feasibility Study on the Use of Foil Bearings in Cryogenic Turbopumps

This paper deals with the feasibility of replacing in cryogenic turbopumps the presently used ball bearings, which have a short life, with fluid film bearings of unlimited life. The particular family of bearings considered is that of compliant foil bearings which, in addition to long life can be given any desired stiffness and damping characteristics so as to impart optimum rotordynamic characteristics to the turbopump rotor. The chief objective of the study was to investigate foil bearings for use in future earth-to-orbit (TEO) rocket engines and was to be accomplished by studying an existing engine which could be considered representative of future engines. A design parameter of a liquid oxygen (LOX) pump of the space shuttle main engine (SSME) was chosen as a prototype to conduct this feasibility study. A parametric study of several journal and thrust bearings was conducted to determine the best geometries and the best levels of structural stiffness as well as the foil materials suitable for a cryogenic lubricant. Speeds up to 30,000 rpm were considered, as well as the special problems attending start-and-stop operations of the turbopump. Three-pad journal bearings and eight-pad thrust bearings, both operating on a combination of hydrodynamic-hydrostatic principles, were shown to meet the operating requirements of the LOX turbopump. The system includes the installation of a stored high pressure LOX to guarantee rub-free conditions during start-and-stop operation.